In practice, this cavity is essentially formed in the primary coaxial part, and it is closed by the centring means. This is the case for example in the specification of German published patent application DE 3 931 429A. In that document, the friction means are also mounted in the same cavity. As a result, in that arrangement, the friction means are subjected to the lubricating action of the grease that is present in the cavity, which is detrimental to their effectiveness. A further result is that the products of wear in the friction means will inevitably contaminate the grease, and this again is detrimental to the effective action of the grease.